Nuclear magnetic resonance (NMR) volumetric blood flow signals have been obtained with coils placed over the region of the heart, the carotid artery, middle cerebral artery, popliteal artery, posterior tibial artery and from the brachial and ulnar arteries. A newly developed ranging technique now makes it possible to record signals from selected vessels or regions of tissue. With this method, it has been possible to obtain volumetric flow signals from tube diameters of 2.9 mm spaced up to 7 cm from the detection system with blood flows of 80 mm/min. This newly developed ranging system has the capability of erasing all the NMR signals except at the region of recording and it is possible to select specific areas for either arterial or venous flow detection. Studies in two patients with unilateral cerebral vascular insufficiency secondary to middle cerebral artery occlusion have demonstrated a marked decrease in the signals on the affected side in contrast to the contralateral normal side. The NMR blood flowmeter can measure volumetric flow quantitatively and on-line, it is noninvasive and the system does not require contact with the subject and non-magnetic materials such as clothes, bandages, and casts do not interfere with measurement. The tissue is transparent to the low frequency magnetic fields and relatively transparent to the radio frequency magnetic field used in the system. Experimental studies in the chimpanzee will be used to determine the optimum sensitivity of the system. Followed by studies in the normal adult humn volunteer, and then in patients with cerebral ischemia secondary to hemodynamic insufficiency.